Natural gas is an important resource widely used as energy source or as industrial feedstock used in, for example, manufacture of plastics. Comprising primarily of methane, natural gas is a mixture of naturally occurring hydrocarbon gases and is typically found in deep underground natural rock formations or other hydrocarbon reservoirs. Other components of natural gas include, but are not limited to, ethane, propane, carbon dioxide, nitrogen, and hydrogen sulfide.
Typically, natural gas is transported from source to consumers through pipelines that physically connect a reservoir to a market. Because natural gas is sometimes found in remote areas devoid of necessary infrastructure (i.e., pipelines), alternative methods for transporting natural gas must be used. This situation commonly arises when the source of natural gas and the market are separated by great distances, for example a large body of water. Bringing this natural gas from remote areas to market can have significant commercial value if the cost of transporting natural gas is minimized.
One alternative method of transporting natural gas involves converting natural gas into a liquefied form through a liquefaction process. Because natural gas exists in vapor phase under standard atmospheric conditions, it must be subjected to certain thermodynamic processes in order to be liquefied to produce liquefied natural gas (LNG). In its liquefied form, natural gas has a specific volume that is significantly lower than its specific volume in its vapor form. Thus, the liquefaction process greatly increases the ease of transporting and storing natural gas, particularly in cases where pipelines are not available. For example, ocean liners carrying LNG tanks can effectively link a natural gas source with a distant market when the source and market are separated by large bodies of water.
Converting natural gas to its liquefied form can have other economic benefits. For example, storing LNG can help balance out periodic fluctuations in natural gas supply and demand. In particular, LNG can be more easily “stockpiled” for later use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
In order to store and transport natural gas in the liquid state, the natural gas is typically cooled to −160° C. at near-atmospheric vapor pressure. Liquefaction of natural gas can be achieved by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems).
There are growing efforts to develop floating liquefied natural gas (FLNG) technologies that would enable operations of water-based LNG processing facilities. Such a facility could float above an offshore natural gas field, where it can produce, liquefy, store and transfer LNG at sea before shipping the LNG directly to markets.
Various aspects of the floating facility will be subjected to inspection (e.g., checking for inner tank leakage, effectiveness of insulation, etc.) and/or maintenance, which may require physical entry into FLNG LNG storage tanks. Before physical entry is possible, LNG cargo stored in the storage tanks need to be vaporized and recovered after inspection and/or maintenance. While conventional methods for vaporizing and evacuating LNG from LNG storage tanks exist, there may be additional technical challenges to removing residual LNG from FLNG LNG storage tanks due to limited space constraints. Moreover, some conventional methods of LNG do not recover vaporized LNG.